Stabilization device for the dynamic stabilization of vertebrae or bones and rod like element for such a stabilization device

ABSTRACT

A stabilization device includes a first bone anchoring element, a second bone anchoring element, and a rod shaped element for connecting the first and the second bone anchoring elements. The rod-shaped element includes a curved section, that undergoes an elastic deformation under the action of a force acting on the rod-shaped element via an anchoring element. The rod-shaped element and the anchoring elements are arranged so that deformation occurs when a force acts in a main direction of motion of bone parts or vertebra relative to each other, whereas a deformation is suppressed when a force acts in a direction that is essentially perpendicular to the main direction of motion.

REFERENCE TO EARLIER FILED APPLICATIONS

The present invention claims the benefit of the filing date under 35U.S.C. § 119(e) of Provisional U.S. Patent Application Ser. No.60/550,697, filed Mar. 5, 2004, which is hereby incorporated byreference. The present application also claims foreign priority benefitspursuant to 35 U.S.C. § 119(a)-(d) for German Patent Application 10 2004010 844.7, filed Mar. 5, 2004, in Germany.

BACKGROUND

The present invention relates to a stabilization device for the dynamicstabilization of vertebrae or bones, and a rod-like element used in thisstabilization device.

European Patent Application, EP 1 188 416 A1, discloses a dynamicstabilization device for stabilizing neighboring thoracic vertebra. Thedevice comprises two pedicle screws and a cable-like strap (31), whichis attached to the receiving part of the pedicle screw by a clampingscrew. The device also contains a support element (3) in the form of apressure-resistant body that is mounted on the strap.

U.S. Patent Publication 2003/0109880 A1 describes a dynamicstabilization device for vertebrae which comprises a first and a secondscrew to be anchored in a vertebra. A coil spring (11) connects an upperportion of the first screw and the upper portion of the second screw.

U.S. Pat. No. 5,415,661 describes an implantable spinal assist devicewhich includes a body composed of a composite material made up of afiber material interspersed in a matrix material. The body is curvedalong its longitudinal axis and comprises a curved center sectionunattached to a motion segment unit and a pair of opposed terminalsections. Similarly, U.S. Patent Publication 2003/0191470 A1 discloses adynamic fixation device. The device includes a flexible portion and twoends that are adapted for connection to pedicle screws. In both of thesereferences, the curved elastic rod, which is anchored at its ends onneighboring vertebrae on the same side of the spinal column by means ofanchoring elements with a shaft of the pedicle screws. Therefore, thecurved elastic rods face in the anterior/posterior direction. Thecurvature fulfils the function of a spring rod.

The known stabilization devices each use two of these stabilizationdevices for stabilizing two neighboring vertebrae with the stabilizationdevices being anchored to the right and to the left of the middlelongitudinal axis of the spinal column.

U.S. Pat. No. 6,440,169 B1, describes a stabilization device for thespinal column, which comprises an elastic body in the form of a leafspring which extends between anchoring points in neighboring vertebraeand is shaped such that it forms the wall of an orifice, whereby theorifice faces in the anterior/posterior direction with respect to thebody of the patient and is closed in the medial/lateral direction.

It is it is desirable to support the disk in a defined fashion withrespect to the transmitted forces and to control the motion by means ofa posterior dynamic stabilization device, especially in the presence ofa damaged or an artificial intervertebral disk. In this regard, aflexion and an extension of the spinal column or of the elementcontaining the respective intervertebral disk, as shown in FIG. 8 a, isdesired, whereas lateral translational motion as well as a torsionalmotion around the middle longitudinal axis of the spinal column, asshown in FIG. 8 b, is undesired.

BRIEF SUMMARY

It is therefore the object of the present invention to provide a dynamicstabilization device for stabilizing and limiting the motion ofneighboring vertebrae or bones, which device is simple in design andpermits a limited degree of flexion and extension yet prevents lateraltranslational motion and a rotational motion of the vertebrae or bonesrelative to each other.

The present invention comprises a stabilization device with a first boneanchoring element, a second bone anchoring element and a rod shapedelement for connecting the first bone anchoring element and the secondbone anchoring element which comprises a curved section that undergoeselastic deformation under the action of a force acting on a rod shapedelement via the bone anchoring element.

The present invention also comprises a stabilization device with a firstbone anchoring element, a second bone anchoring element, a third boneanchoring element, a fourth bone anchoring element, a first rod-shapedelement that is connect to the first or second bone anchoring elementand to the third or fourth bone anchoring element and a secondrod-shaped element that is connected to the second or first boneanchoring element and to the fourth or third bone anchoring element,whereby the points of connection of the bone anchoring elements to therod-shaped elements define a plane and the rod-shaped elements eachcomprise a curved section which undergoes an elastic deformation underthe action of a force acting on the rod-shaped element via an anchoringelement and wherein the rod-shaped elements are arranged so that thecurved section resides within the plane or is parallel to the planedefined by these various components.

The present invention also comprises a dynamic stabilization device forhaving a first bone anchoring element, a second bone anchoring element,and a rod-shaped element that can be connected to the first and to thesecond bone anchoring element, whereby the rod-shaped element includes acurved section (7) between the first and second bone anchoring elements,which undergoes an elastic deformation under the action of a forceacting on the rod-shaped element via the anchoring element; and whereinthe curved section, in the operational state of the stabilizationdevice, resides within a plane that is essentially perpendicular to themedian plane of the human body.

The invention is advantageous in that a desired limitation of motion canbe achieved in a simple fashion by selecting suitably dimensionedrod-shaped elements. Moreover, the rod-shaped elements of the presentinvention are easy to manufacture. Furthermore, the stabilization devicecan also be used with artificial intervertebral disks.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The presently preferred embodiments, together with furtheradvantages, will be best understood by reference to the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of the stabilization device according to a firstembodiment of the invention viewed from the posterior side of the spinalcolumn;

FIG. 2 a shows a perspective view of a rod-shaped element of thestabilization device according to FIG. 1;

FIG. 2 b shows a perspective view of the stabilization device accordingto FIG. 1 viewed from above with a preferred implementation of therod-shaped elements (the upper two bone anchoring elements not shown);

FIGS. 2 c and 2 d show a perspective view of modifications of therod-shaped element to be used in the stabilization device according toFIG. 1;

FIG. 3 shows a partial sectional view of an anchoring element foranchoring the rod-shaped element of FIG. 2 according to a firstembodiment;

FIG. 4 shows a partial sectional exploded view of an anchoring elementfor anchoring the rod-shaped element of FIG. 2 according to a secondembodiment;

FIG. 5 shows a partial sectional view of the anchoring element of FIG. 4connected to a rod-shaped element;

FIG. 6 a shows a schematic illustration of the mobility of a rod-shapedelement of the stabilization device of FIG. 1;

FIG. 6 b shows a schematic illustration of the function of thestabilization device according to the first embodiment;

FIG. 6 c shows a modification of the stabilization device according tothe first embodiment;

FIG. 7 shows a modification of the stabilization device of the firstembodiment;

FIG. 8 a shows a section of the spinal column viewed from the side inflexion and extension; and

FIG. 8 b shows a section of the spinal column viewed from behind,showing lateral translation motion as well as torsional motion aroundthe longitudinal axis of the spinal column.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

The invention and various embodiments thereof are presented in FIGS. 1to 7 and the accompanying descriptions wherein like numbered items areidentical.

As is seen in FIG. 1, the stabilization device comprises a first and asecond rod-shaped element, 1,1′. These two rod-shaped elements are eachconnected to two bone anchoring elements, which are anchored in thepedicles of two neighboring vertebrae, 4,5. In this particular example,pedicle screws are used as the bone anchoring elements. The firstpedicle screw 2 of rod-shaped element 1 is anchored to the right pedicleof the lower vertebra, 4, whereas the second pedicle screw, 3, ofrod-shaped element 1 is anchored to the left pedicle of the uppervertebra, 5 Symmetrical to this arrangement, the first pedicle screw,2′, of rod-shaped element 1′ is anchored to the left pedicle of thelower vertebra, 4, whereas the second pedicle screw, 3′, of rod-shapedelement 1′ is anchored to the right pedicle of the upper vertebra, 5. Inthis embodiment, the rod-shaped elements, 1,1′ intersect at a point, K,within the plane of symmetry M (shown in FIG. 6 b).

The first rod-shaped element, 1, is in the form of a curved rod with arectangular cross-section as seen in FIG. 2 a. Rod-shaped element 1comprises a first straight section, 6, of length L1, and, adjacent toit, a curved section, 7, of length L2, and, adjacent to it, anotherstraight section 8 of length L3. The middle longitudinal axes, M1 andM2, of the straight sections intersect and form an obtuse angle, α. Inthe embodiment shown, the length, L1, of the first straight section, 6is approximately three times the length, L3, of the second straightsection, 8. The rod-shaped element has a broad side, B, and a narrowside, S. The curvature extends over the broad side, B, of the rod-shapedelement. The flexural strength due to a force acting perpendicular tothe broad side, B, of the rod-shaped element is smaller than theflexural strength due to a force acting perpendicular on its narrowside, S. Moreover, due to the lever effect resulting from the differencein the lengths, L1 and L3, of the straight sections, 6, 8, the flexuralstrength of the rod-shaped element with respect to a force actingperpendicularly to the rod-shaped element on the long section, 6, issmaller than the flexural strength with respect to a force acting on theend of the short section, 8, in an orthogonal direction.

The rod-shaped element thus possesses oriented flexural strength thatcan be adjusted during the manufacture as desired by selecting thecross-section, curvature, and length of the rod-shaped element toachieve the desired properties.

The rod-shaped element 1 can be made from a body-compatible materials,preferably from one piece of such material. The body compatible materialcomprises stainless steel, titanium alloys, nickel-titanium alloys,nitinol, chrome alloy, cobalt chrome alloys, shape memory alloys,materials with super elastic properties, carbon reinforced composites,silicone, polyurethane, polyester, polyether, polyalkene, polyethylene,polyamide, poly(vinyl) fluoride, polyetheretherketone (PEEK), orpolytetrafluoroethylene (PTFE).

In its simplest embodiment, the second rod-shaped element, 1′, isattached in mirror-like symmetry to the first rod-shaped element, 1,such that the curved sections face in opposite directions.

FIG. 2 b shows the stabilization device of FIG. 1 from above with apreferred implementation of rod-shaped elements, 1, 1′. In this FIG. 2b, the respective upper bone anchoring elements are not shown. In anarea, 1 a, 1 a′, adjacent to the section with which they are anchored inthe anchoring element, rod-shaped elements 1, 1′ are curved to extendout of the plane defined by the connection points so that they areguided past each other without interfering with each other in theoperational state of the stabilization device. This configurationenables all the connection points of the bone anchoring elements of therod-shaped elements to reside in a plane that is orthogonal to themedian plane indicated in FIG. 8 a.

FIG. 2 c shows a modification of the rod-shaped element which can beused in the stabilization device of FIG. 1. The rod-shaped elementaccording to FIG. 2 c differs from the rod-shaped element shown in FIG.2 a because the cross-section is spherical, instead of rectangular. Asis shown in FIG. 2 d, the diameter of the rod-shaped element may vary.For example, it can increase from the free end of the first straightsection 6 towards the free end of the second straight section 8.

As is shown in FIG. 3, in a first embodiment, the rod-shaped element, 1,1′, is fixed in a monoaxial pedicle screw. This screw comprises athreaded shaft, 10, with a bone thread and a receiving part, 11, that isrigidly connected thereto. Receiving part 11 is provided to beessentially cylindrically symmetrical in shape and includes a recess,12, which extends from its free end facing away from threaded shaft 10.The receiving part 11 has a rectangular cross-section that isdimensioned so that rod-shaped element 1, 1′ can be placed therein andstill remain capable of shifting in the direction of its longitudinalaxis. The narrow side, S, of rod-shaped element 1 rests in the base ofthe recess 12 of the receiving part. The receiving part also includes anexternal thread, 13, that extends from its free end and has apre-determined length which is dimensioned such that, when therod-shaped element inserted, the external thread extends to below theupper side thereof. A nut, 14, that can be screwed onto the externalthread 13 is provided for fixing the rod-shaped element in the receivingpart.

In a further embodiment, a polyaxial screw is used as the bone anchoringelement for connecting to the rod-shaped element, 1, 1′, is as shown inFIGS. 4 and 5. This polyaxial screw comprises a screw element, 15, witha threaded shaft, 16, with a bone thread, and a spherical segment-shapedhead, 17, with a recess for screwing-in (not shown). Furthermore, areceiving part, 18, that can be connected to screw element 15 is shown.The screw element 15 pivots relative to the receiving part 18. Thereceiving part 18 is provided to be essentially cylindrical in shape andcomprises at its one end a first bore, 19, that is aligned axially withthe screw element 15. The first bore 19 has a diameter that is smallerthan that of head 17 of the screw element 15. Receiving part 18 furthercomprises a coaxial second bore, 20, that is open on its end opposite tofirst bore 19 and whose diameter is sufficiently large to enable thescrew element, 15, to be guided through the open end with its threadedshaft through first bore 19 and with head 17 to the base of second bore20. A spherically shaped section or a ledge, 21, is provided in thereceiving part for abutment of head 17 between the first bore, 19, andthe second bore, 20. It will be appreciated by those skilled in the artthat any shaped structure can be used provided that it holds the head ofthe screw element in the bore.

The receiving part 18 further comprises a recess 22, which is arrangedsymmetrically with respect to the middle of the receiving part and has arectangular cross-section for receiving the rod-shaped element, 1, 1′,and through which two free legs, 23, 24, are formed. In an area on thereceiving part 18, adjacent to the free end, legs 23, 24, there may bean external thread, 25, and an internal thread, 26.

In addition, the polyaxial screw may include a pressure element, 27,that is capable of pressing onto the head 17 of the screw. Pressureelement 27 comprises a coaxial bore, 28, for insertion of a screw driveron one side, and, at its side facing head 17, a spherical recess, 29,for receiving the receiving head 17. At its side facing away from thehead, pressure element 27 comprises a recess, 30, with a rectangularcross-section for receiving the rod-shaped element, 1, 1′. The width ofrecess 30 is just slightly larger than the narrow side, S, of rod-shapedelement 1, 1′ such that the rod-shaped element can be introduced withits narrow side, S, towards the base of the recess so that it is capableof shifting in longitudinal direction within the recess. The depth ofrecess 30 is preferably slightly smaller than height B of the rod-shapedelement. Although shown in this manner, it will be appreciated by thoseskilled in the art that the pressure element can be of other shapes solong as it is capable of pressing onto the head of the screw.

Moreover, an internal screw 31 can be screwed onto the legs 23, 24 forfixing rod-shaped element 1, 1′ in the receiving part as well as forfixing the angle position of screw element 15 relative to the receivingpart. A nut 32 can be screwed onto the outside of legs 23, 24 to securethe receiving part 18.

In operation, the pedicle screws, 2, 3, 2′, 3′, are first screwed intothe vertebrae, 4, 5, to be connected by the stabilization device. In thecase of the monoaxial screws shown in FIG. 3, the threaded shafts, 10,are screwed in until the receiving parts, 11, of the pedicle screws, 2,3 or 2′, 3′ to be connected to a rod-shaped element 1, 1′ are alignedwith respect to each other such that the rod-shaped element can beintroduced without jamming. The rod-shaped element is then inserted intothe receiving part and fixed into the receiving part by screwing-on nut14.

In the case of the polyaxial screws, shown in FIGS. 4 and 5, screwelement 15 is first inserted into receiving part 18, then followed bythe pressure element 27. Then the screw element 15 is screwed into thepedicle as described above. Subsequently, the rod-shaped element 1 or 1′is inserted into the receiving part. Due to the flexible connectionbetween the head 17 and receiving part 18, the receiving parts 18 of twopedicle screws 2, 3 or 2′, 3′ to be connected by the rod-shaped element,1 or 1′, align themselves correctly with respect to the rod, which is ofadvantage in the intersecting arrangement of the rod-shaped elements.Subsequently, the receiving parts are fixed relative to the head and therod by means of the internal screw and the nut.

FIGS. 6 a and 6 b show a schematic illustration of the function of thestabilization device. The rod-shaped element 1 shown in FIG. 6 a with noload can be likened to an angle lever with a long lever arm, 6, and ashort lever arm, 8. In the presence of load due to flexion or extensionof the spinal column segment, a small force component acting on theanchoring points orthogonal to long lever arm 6 leads to the elasticdeformation of rod-shaped element 1, by displacing the long lever armand increasing or reducing the size of the radius of curvature relativeto the resting position upon flexion or extension, respectively.

The continuous lines and the dashed lines in FIG. 6 b show rod-shapedelements, 1, 1′, in a first state and in a second state, respectively,with the latter corresponding to a flexion of the spinal column segment.In the first state, pedicle screws 2′, 3, to which rod-shaped element 1is connected, are at a distance, H, from each other in the direction ofthe middle longitudinal axis of the spinal column. Flexion of the spinalcolumn segment leads to the bending of rod-shaped element 1, whereby theradius of curvature increases and therefore the distance between thepedicle screws from each other in the direction of the middlelongitudinal axis increases to H′. In contrast, an extension of thespinal column segment causes the radius of curvature of the rod-shapedelements to decrease. This causes the distance between the pediclescrews to decrease. (not shown).

When a force component acts perpendicularly to the short lever arm, itis more difficult or even impossible for the curvature of the rod-shapedelements to change because a greater force is required. This prevents alateral translational motion. Due to the elongated implementation of therod-shaped elements, there is also a high degree of rotational stabilitywith respect to a rotational motion of the vertebrae relative to eachother.

FIG. 6 c shows a modification of the stabilization device according toFIG. 1. Instead of rigid rod-shaped elements 1, 1′ rod-shaped elements1″, 1′″ are used which have a spring-like flexible section 6″, 6′″.Preferably the spring-like section 6″, 6′″ is the longer straightsection. Flexibility is achieved, e.g. by a helical recess 60 creatinghelical spring-like windings 61. The rod-shaped element may be hollow.The spring-like section can be achieved with another construction.

FIG. 7 shows a second embodiment of the instant invention. Thestabilization device of this embodiment comprises to two motionelements, that extend between three vertebrae, 4, 5 and 5′. Thisembodiment comprises rod-shaped elements, 101, 101′, whose shape is“u-shaped.” That is the rod-shaped element is identical to thatgenerated by mirroring a rod-shaped element, 1, 1′, according to thefirst embodiment on a plane that stands orthogonal on the free end ofthe short section. Rod-shaped elements 101, 101′ each are anchored attheir ends in the pedicles on the same side of the spinal column bymeans of pedicle screws 2, 2″ or 2′, 3″, between two vertebra, 4, 5′which have a intermediate vertebrae 5 between them. The middle sectionsof the rod-shaped elements 101 and 101′ are also anchored in thepedicles of the intermediate vertebra 5 on the opposite side of thespinal column by means of pedicle screws 3 or 3′. The rod-shapedelements 101 and 101′ can be constructed from a unitary rod or frommulti-segments which are joined together to form a unitary rodstructure.

Modifications of the embodiments described above are also possible. Forexample, the lengths of the individual sections of the rod-shapedelements can be selected according to the dimensions of the vertebrae tobe connected. The invention is not limited to the rod-shaped elements ofthe stabilization devices of the first embodiment comprising a long anda short straight section. There can also be only one long and one curvedsection. In the stabilization device according to the second embodiment,the straight sections can differ in length, however in this embodimentit is important that the rod-shaped element is curved between the oneattachment point and the other attachment point so that the rod-shapedelement effects a change in the distance of the attachment points underthe action of flexural moments.

Moreover, the features of the various embodiments described above can becombined with each other. The stabilization device according to thefirst embodiment can also comprise rod-shaped elements with a circularcross-section. A square cross-section shall also be possible, thoughthis reduces one degree of freedom in the selection of theflexural-elastic properties of the rod-shaped element.

Instead of the monoaxial screws and polyaxial screws described above,monoaxial screws and polyaxial screws can be used, in which the fixationof the head or the rod occurs by different means. Hooks may also be usedfor fixation of the rod-shaped element.

Although the drawing and descriptions describe the stabilization devicefor the spinal column, the invention should not be so limited. Theinstant invention can also be used to stabilize bones.

The embodiments described above and shown herein are illustrative andnot restrictive. The scope of the invention is indicated by the claims,including all equivalents, rather than by the foregoing description andattached drawings. The invention may be embodied in other specific formswithout departing from the spirit and scope of the invention.

1. A stabilization device for the dynamic stabilization of vertebrae orbones, comprising: a first bone anchoring element for anchoring in afirst bone part or vertebra; a second bone anchoring element foranchoring in a second bone part or vertebra; a generally L-shaped rodcomprising a first generally straight end portion having a first freeend, a curved portion having a first end extending from the firstgenerally straight portion, and a second generally straight end portionextending from a second end of the curved portion, the second generallystraight end portion having a second free end, wherein the entire rodfrom the first free end to the second free end is generally L-shaped andwherein the second generally straight portion is longer than the firstgenerally straight end portion; wherein the first bone anchoring elementis attached directly to the first generally straight end portion;wherein the second bone anchoring element is attached directly to thesecond generally straight end portion; and wherein the curved sectionelastically deforms when a force acts on the rod by any one of the firstbone anchoring element and second bone anchoring element.
 2. Astabilization device according to claim 1, wherein the first and thesecond bone anchoring elements are arranged diagonally opposite fromeach other relative to a main direction of motion.
 3. A stabilizationdevice according to claim 1, wherein the bone anchoring elements areanchored in diagonal-opposite pedicles of two neighboring vertebrae. 4.A stabilization device according to claim 1, wherein said stabilizationdevice further comprises a third and a fourth bone anchoring element anda second rod connecting the third and the fourth bone anchoring element,said third and fourth bone anchoring element with said second rod beingarranged symmetrically to the first and the second bone anchoringelement with the L-shaped rod.
 5. A stabilization device according toclaim 4, wherein said L-shaped rod and said second rod each intersectthe plane of symmetry at least once.
 6. A stabilization device accordingto claim 4, wherein the number of the bone anchoring elements is amultiple of two and the rods each connect more than two bone anchoringelements.
 7. A stabilization device according to claim 1, wherein theL-shaped rod has a rectangular cross-section.
 8. A stabilization deviceaccording to claim 1, wherein the L-shaped rod has a squarecross-section.
 9. A stabilization device according to claim 1, whereinthe L-shaped rod has a circular cross-section.
 10. A stabilizationdevice according to claim 1, wherein the first bone anchoring elementcomprises an anchoring section for anchoring in a bone and a receptacleconnected thereto which has a recess having a cross-section, wherein therecess cross-section matches the cross-section of the L-shaped rod. 11.A stabilization device according to claim 10, wherein the anchoringsection is rigidly connected to the receptacle.
 12. A stabilizationdevice according to claim 10, wherein the anchoring section and thereceptacle have opposed contacting surfaces that move relative to eachother to permit pivoting.
 13. A stabilization device according to claim1 wherein at least a portion of the second generally straight endportion comprises a helical slot.
 14. A method of dynamicallystabilizing vertebrae of a spinal column or bones with a dynamicstabilization device comprising a first bone anchoring element and asecond bone anchoring element for anchoring in a first bone part or avertebra, a third bone anchoring element and a fourth bone anchoringelement for anchoring in a second bone part or vertebra, a firstrod-shaped element comprising a first portion, a second portionconfigured transverse to the first portion, and a curved portionconnecting the first portion and the second portion and configured toelastically deform when a force acts on at least one of the firstportion and the second portion, a second rod-shaped element comprising afirst portion, a second portion configured transverse to the firstportion, and a curved portion connecting the first portion and thesecond portion and configured to elastically deform when a force acts onat least one of the first portion and the second portion, the methodcomprising: anchoring the first bone anchoring element in the first bonepart or vertebrae; anchoring the second bone anchoring element in thefirst bone part or a vertebra; anchoring the third bone anchoringelement in the second bone part or vertebra along a length of the spinalcolumn relative to the first bone anchoring element; anchoring a fourthbone anchoring element in the second bone part or a vertebra along thelength of the spinal column relative to the second bone anchoringelement; fixing the first portion of the first rod-shaped elementdirectly to a rod receiving part of the first bone anchoring element toprevent movement of the rod receiving part along the first rod shapedelement; fixing the second portion of the first rod-shaped elementdirectly to a rod receiving part of the fourth bone anchoring element toprevent movement of the rod receiving part along the first rod shapedelement; fixing the first portion of the second rod-shaped elementdirectly to a rod receiving part of the second bone anchoring element toprevent movement of the rod receiving part along the second rod shapedelement; fixing the second portion of the second rod-shaped elementdirectly to a rod receiving part of the third bone anchoring element toprevent movement of the rod receiving part along the second rod shapedelement; wherein when fixed as above the first bone anchoring element ismoveable along the length of the spinal column relative to the thirdbone anchoring element due to flexing of at least one of the first andthe second rod-shaped elements; and wherein when fixed as above thesecond bone anchoring element is moveable along the length of the spinalcolumn relative to the fourth bone anchoring element due to flexing ofat least one of the first and the second rod-shaped elements.